1. Field of the Invention
This invention relates to improvements in a control system for a vehicular power plant including an automatic transmission and an exhaust brake, and more particularly to a device for temporarily putting the exhaust brake into an inoperative condition to suppress a shift shock due to the application of the exhaust brake.
2. Description of the Prior Art
Hitherto, an exhaust brake has been proposed and put into practical use, in which exhaust gas from an internal combustion engine is blockable to improve an engine braking. An example of control systems for the exhaust brake is disclosed in Japanese Patent Provisional Publication No. 2-78741, in which the operation of the exhaust brake is stopped for a predetermined time after a transmission selector for an automatic transmission has been changed to have a position for application of the engine brake. Thus, during an automatic transmission shifting upon a driver's selecting operation, the exhaust brake is released from being applied thereby preventing an excessive shift shock from occurring.
However, drawbacks have been encountered in the above conventional exhaust brake control system, as discussed below. That is to say, although the conventional control system has an advantageous effect to reduce the shift shock during the automatic transmission shifting upon the driver's selecting operation, it has difficulties that a strong shift shock occurs when a shiftdown is automatically made according to a control program of a control unit. More specifically, assuming that the vehicle speed is considerably high when a driver's selecting operation is made, for example, from a D-range to a 1-range for engine braking, the vehicle speed cannot be immediately lowered to the first speed, in which a shift from the third to the second speed is first made and then another shift from the second to the first speed is subsequently made. In such a conventional control system, the exhaust brake is in an operative condition when the third to second speed shift and the second to first speed shift are made, thereby causing strong shift shocks. Otherwise, when the vehicle speed is relatively low when the driver's selecting operation is made, the third to second speed shift is carried out simultaneously with the selecting operation, in which the shift shock is softened because the exhaust brake is temporarily released from being applied. However, when the second to first speed shift is carried out, the exhaust brake is in operation thereby causing a strong shift shock.